From test case: https://qatestrail.hq.unity3d.com/index.php?/cases/view/696
If user declines to update the obsoleted scripts during importing, the menu item "Run API Update" stays greyed out.
The menu item will go back to normal if the user restarts the Editor.

Repro:
1. Create a new project (it has to be a project never has API update invoked before)
2. Import the script from test case or attachment
3. When the "API update" window appears, choose "No Thanks"
4. Go to Assets (Menu bar) -> Run API Updater...
(optional)
5. Restart Unity Editor with the same project
6. "API Updater" appears and choose "No Thanks."
7. Go to Assets (Menu bar) -> Run API Updater...

Expected: User can immediately use API Updater again after initial rejection.